An alkali metal hydrogencarbonate such as sodium hydrogencarbonate is produced by crystallization from an aqueous solution. For example, carbon dioxide is blown into an aqueous solution containing sodium ions to obtain sodium hydrogencarbonate crystals formed by a reaction. As the solution containing sodium ions, an aqueous solution containing sodium carbonate or sodium hydroxide, or so-called ammonia brine obtained by blowing ammonia into a sodium chloride solution, is employed.
For example, sodium hydrogencarbonate is usually difficult to grow to large particles and tends to have a small average particle size, since nuclei tend to be formed in a large amount during crystallization, whereby the proportion of fine particulate crystals will increase, and the total number of crystals will increase. Formation of nuclei is likely to be remarkable if it is attempted to obtain large particles and is considered to be caused by peeling of seed crystals from the surface of crystals by a rapid flow of the mother liquor at the surface of the particles.
And, if such fine particulate or small crystals increase, it is likely that the yield of large crystals will decrease, and further the subsequent operation such as separation, washing, drying or classification of the solid tends to be difficult. Further, large size crystals are desired, for example, in an application where flowability as a powder product is required, and it is an object for a crystallization process how to obtain large crystals.
In a conventional crystallization process, in order to increase the size of crystals to be precipitated, a method of using a crystal habit modifier has been, for example, employed. However, if a crystal habit modifier is used, the crystal habit is likely to be changed, whereby the crystal state may be changed, and since the effects will be obtained under specific conditions, it has been difficult to use such a method in combination with other methods. Further, the crystal habit modifier may remain in crystals thereby to lower the purity of the crystals, whereby the application of the product tends to be limited.
Further, a method is also known wherein the crystallization load i.e. the amount of crystals formed per unit time and unit volume is substantially reduced to lower the super saturation degree of the crystal component precipitated in the mother liquor thereby to minimize formation of nuclei. However, in this method, the crystallization load per unit time and unit volume of the crystallization equipment is required to be low, whereby the productivity of the equipment used to be low in many cases.
Further, in order to lower the super saturation degree of the crystal component precipitated in the mother liquor, there is a method of adding a third component (such as sodium chloride or ethyl alcohol) to lower the solubility of the alkali metal hydrogencarbonate. However, for its separation from the mother liquor, a cake is required to be sufficiently washed, and the above third component may still remain in the crystals. Further, in a case where sodium chloride is to be added, there will be a limitation to the material for the equipment to prevent corrosion, and in a case where an inflammable organic solvent such as ethanol is to be added, an operational limitation will be involved taking the inflammability into consideration, whereby the operation tends to be complex.
Further, the particle size of crystals may be made large also by a method of removing formed fine particulate crystals out of the system. However, if this method is carried out as a continuous operation, it will be difficult from the viewpoint of the equipment to selectively remove fine particles in such a large amount sufficient to increase the crystal size, whereby there is a problem such that it is difficult to carry out treatment of fine particulate crystals sufficiently. Further, an equipment to treat removed fine particles will be required, and the yield tends to be low. Thus, it was not possible to industrially produce large crystals in good yield solely by this method.
Further, a process for producing an alkali metal hydrogencarbonate is disclosed which comprises reacting an aqueous solution containing alkali metal ions with carbon dioxide to precipitate crystals, wherein an operation to dissolve and remove fine particles of an alkali metal hydrogencarbonate by bringing the concentration of the alkali metal hydrogencarbonate dissolved in a liquid to a level of at most the saturation solubility with respect to the entire crystallization liquid in the crystallization equipment, is intermittently repeated (Patent Document 1). However, blowing of carbon dioxide gas is required to be intermittently switched to air, whereby the operation efficiency is poor and an investment is required to control the equipment.
Patent Document 1: WO01/14254 (Claims)